Frequency changer



June 21, 1960 R. D. BURNHAM FREQUENCY CHANGER Filed July 1, 1957 3e 5/ 50 F 1g.!

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INVENTOR.

ROBERT D. BURNHAM 0 a; alum/V ATTORNEYS June 21, 1960 R. D. BURNHAM 2,942,171

FREQUENCY CHANGER Filed July 1, 1957 3 Sheets-Sheet 2 F lg. 7

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INVENTOR. ROBERT D. BURNHAM pfylt BIA/WM ATTORNEYS June 21, 1960 R. D. BURNHAM FREQUENCY CHANGER 3 Sheets-Sheet 3 Filed July 1, 1957 POSITIVE VOLTAGE NEGATIVE Fig I3 \/\/\REsuLTANT voLIAcE XXA I: Fig. I4

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H G INVENTOR. ROBERT D. BURNHAM f RESULTANT VOLTAGE ATTORNEYS T/ME- Fig. 16

II I I United States Patent FREQUENCY CHANGER Robert D. Burnham, 516 Langfitt S't., Vermilion, Ohio Filed July 1, 1957, Ser. No. 669,248

8 Claims. (Cl. 321-7) This invention relates to alternating current wave changers and more particularly to an improved frequency changer. The device is an improvement of the type of frequency multiplier disclosed by W. H. Bussey in his United States Patent No. 2,470,975, issued May 24, 1949, under the title Electrical Apparatus.

In general, the Bussey apparatus comprises a'pair of ferromagnetic bodies which are joined together by a series of magnetic strips. These strips or teeth form parallel magnetic paths between the two bodies.

A polyphased input winding is wrapped about one of the bodies in a manner which is quite similar to the winding of an induction motor. The input winding is constructed so that a rotating magnetic field is established. According to the Bussey teaching this magnetic field is of suificient strength to cause magnetic saturation in the magnetic paths.

In the Bussey device there is a relatively small zone perpendicular to the magnetic axis where the magnetic saturation in the paths are changed from saturation in one direction through zero to saturation in the opposite direction. During the period of time when this change is taking place a voltage is induced in output coils which are wrapped around the previously described teeth. This voltage is induced due to the changing magnetic flux within the paths defined by the teeth.

The Bussey teaching depends on magnetic saturation in the magnetic paths. Magnetic saturation creates many problems. Among these are excessive heating, poor efficiency, noise, localized heating problems, a poor power factor, sensitivity to line voltage unbalance, and reflection of time harmonics of an electrical supply line.

It has been discovered that'an improved frequency multiplier can be constructed by designing a device in which the magnetic paths are nonsaturated. .The frequency multiplication is accomplished by means of rate of change of magneticfiux in the magnetic paths and it does not relyon saturation of its operation.

It is believed that some prior proposals have relied on a theory which is incorrect. This incorrect theory' has led to a belief that there must be magnetic saturation in the magnetic paths. An over simplification of the correct explanation for the operation of'this device is that it depends on Faradays law; Since this is the true manner in which the device operates it becomes apparent that a frequencymultiplier can be constructed with unsaturated magnetic paths.

Accordingly, one. of the principal objects of the invention is to provide a novel and improved frequency multiplier in which relatively little heat is developed in the magnetic core and which need not be large and complex in order to dissipate generated heat.

Another object of the invention is to provide a novel and improved frequency changer-which is smooth and quiet in operation.

Still another object of the invention is to provide a novel and improved changer-multiplier wh does not have localized heating.

A further object of the invention is to provide a novel and improved changer multiplier which has a relatively low magnetization current, an improved power factor, and reduced sensitivity to line voltage unbalance.

A still further object of the invention is to provide a novel and improved frequency changer in which reflection of time harmonics to an electrical supply line are reduced.

Yet another object of the invention is to provide a novel and improved frequency changer in which the output frequency is less than the frequency of the input current.

A still further object of the invention is to provide a novel and improved adjustable frequency changer wherein the input frequency may be either stepped up or stepped down and wherein the frequency change is adjustable.

Still another object of the invention is to provide a novel and improved frequency changer in which a sign wave in put flow of alternating current is changed to a peaked wave flow of alternating current.

One additional object of the invention is to provide a frequency changer which is a rectifier to change alternating current to direct current.

These listed objects will outline the invention, but other objects and a fuller understanding of the invention may be had by referring to the following description and claims, taken in conjunction with the accompanying drawing, in which:

Figure 1 is a somewhat schematic top plan view of a frequency multiplier made in accordance with the teaching of this invention;

Figure 2 is a wiring diagram showing the connection of the input coils;

Figure 3 is a phase diagram of the current flow in the three-phase embodiment of Figure 1;

Figures 4 through 6 show the location of the revolving magnetic fields at the times T T and T respectively of Figure 3;

Figure 7 is a top plan view of a frequency multiplier showing polyphase output;

Figure 8 is a somewhat schematic front elevational view of an adjustable frequency changer;

Figure 9 is a somewhat schematic sideelevational view of the device of Figure 8;

Figure 10 is a front elevational view of a slightly modified form of the invention wherein the frequency change is from alternating current to pulsating direct current;

Figure 11 is a somewhat schematic side elevational view of the device of Figure 10; I

Figure 12 is a diagrammatic view of theslip rings of the device of Figures 10 and 11 showing the relationship and connections of the slip rings; and,

Figures 13 through 16 are diagrams showing sample wave diagrams obtainablethrough the use of the device shown in Figures 10 through 12 in its pictured and slightly modified form.

Referring to the drawings and to Figure 1 in particular an inner ferromagnetic body portion is shown at 10. An outer surrounding body portion is shown at 11. An annular path defining portion 18 is located between the inner and outer body portions 10, 11. The path defining portion 18 has a plurality of teeth 12 through 17. The teeth 12 through 17 are preferably radially disposed and connect the body portions 10, 11.

The body portions and the teeth are made of ferromagnetic material. It will be noted that the over-all appearance as seem from the plan view of Figure 1 somewhat resembles a spoked wheel. A satisfactory method of constructing a ferromagnetic body is the well known technique of laminating. A series of wheel-shaped sheets of ferrous material capable of carrying an induced magnetism are formed. The sheets are then laminated together to form a magnetic core or body as it has been referred to.

In Figures 1 through 6 of the drawings, a three phase input winding arrangement has been shown. Three input windings 20, 21, 22 are provided. They are wrapped around one of the body portions. In Figure 1 they are wrapped'around the inner body portion 10. Three supply conductors 24, 25, 26 are connected to the input windings 20, 21, 22 respectively. The input conductors 24, 25, 26 are in turn connected to a three phase source of electricpotential, which is not shown. The input windings are joined together by a common conductor 27, Figure 1, or at a junction point indicated by 27, Figure 2.

The input windings must be polyphase. It is possible to use a single phase input current which is changed to two phase by means of a phase shifter.

An output conductor 30 is provided. The output conductor is formed to provide output windings located within the field of magnetic influence of the teeth. The conductor is preferably wrapped about each of the teeth 12 through 17 preferably to provide series connected output windings 31 through 36. It will be noted that the windings on adjacent teeth are wound in opposite directions to provide coils of recessed polarity alternately. Thus the windings 32, 34, 36 are wound in one direction and the windings 31, 33, 35 are wound in the other direction.

Refer-ring to Figures 3 through 6, when it is time T the magnetic poles indicated by N and S are aligned with the teeth 12, 15. At this instant there will be a strong induced magnetism in these two teeth 12,15. This will induce a voltage in coils 31, 34. There is a magnetic component in the paths or teeth 14, 17 and 13, 16. The output voltage at this instant is the vector sum of the voltage in all coils.

At the time T Figure 4, the voltages in coils :13, 14, 16, 17 are small and of opposite polarity to the strong voltage of coils 12, 15. This has the effect of increasing the slope of the resultant-voltage wave which is principally created in coils 12 and 15. Thus, if it is desired to only change the wave form, a one-to-one ratio device may be used to change a sine wave to a peaked wave.

As the magnetic field rotates to the position indicated by T Figure 5, it will be seen that the induced voltages in the teeth will offset one another and the resultant voltage will be 0. At time T Figure 6, it will be seen that the rotating magnetic field lies along the paths defined by teeth 14, 17. Thus at the time T the principal voltage will'be induced in the coils 33, 36 equal in intensity to the voltage induced at the time T but in opposite direction.

It can thus be seen that one-half a wave has been generated between the time T and T In the device of Figure 1 three full waves will be generated for each revolution of the magnetic field. Thus with an input frequency of 60 cycles an output frequency of 180 cycles will be obtained in this particular embodiment.

The induced magnetism in the path defining portion 18 of the ferromagnetic core may be divided into zones. A pair of unsaturated zones are located in the path defining portion. At the time T the tooth 15 will be a north pole and the center of one of the pair of unsaturated zones. The tooth '12 will be a south pole and at the center of the other of the second pair of unsaturated zones. A magnetic neutrality will be found at two locations midway between these poles, 90 in both direc? tions from both poles and of'course the areas of magnetic neutrality will be 180 electromagnetic degrees apart..v

At the time T these areas of magnetic neutrality will lie in the teeth 13, 16. Thus, each of the pair of magnetic zones will have only one polarity at one time.

It is the unsaturated condition of thesemagnetic zones which is one of the improvements over the Bussey device.

Frequency multipliers made in accordance with this teaching have shown outstanding characteristics over the Bussey multiplier. The noise level is materially reduced. The efiiciency is increased and the operating temperature is decreased. A further significant improvement is that the improved multiplier is small and simple as compared with the Bussey device. A further advantage is that one need not exercise undue care in locating the present multiplier in a cool location. Installation is no problem.

The number of magnetic paths in this device is generally an even number. The following table will set out the multiplication of base frequency depending upon the number of magnetic paths and the phase of the series output windings.

Multiple of base frequentcy Phase It will be seen that the device depicted in the drawings falls in the chart under 6 magnetic paths and a single phase multiple of base frequency of 3. seen that the devices of Figures 1 through 7 will produce an output frequency which is an odd multiple.

The drawing and the foregoing description suggest the use of two pole input windings. It Will be apparent however that the input windings may be. arranged with as many poles as are desired. It will also be seen that a plurality of these multipliers may be connected in series, or cascaded, so that the frequency multiplication may take place in two or more stages. If such is the arrangement the frequency multiple will be the product of the multiplication of each series connected or cascaded unit.

Figure 7 discloses an example of how the device of Figure 1 may be modified to provide polyphase output.

The input windings 20', 21, and 22' are split into two portions to straddle two of the teeth. This is done for space considerations caused by a' higher number of teeth for polyphase output and to maintain the relative positions of the magnetic poles.

A first set of teeth 40 through 45 are. provided. A second set of teeth 48 through 53 arealso provided. 'An output conductor 55 is provided. The output conductor 55 is wound around the teeth.40 through 45 to provide a first output phase. A "second output conductor 56 is also provided. The second output conductor-56 is wound around the teeth 48 through 53 to provide a second output phase. It will be seen that the output conductors 55, 56 like the output conductor 30 are wound in opposite directions on successive teeth.

. The device of Figure 7 is shown only to illustrate the construction of a polyphase output. It will be seen that alternate teeth provide the output of one phase while the teeth interspersed between them provide the other output phase. The teeth :of the second phase are equally spaced between the teeth of the first phase. It a thirdphase is to be provided three sets of teeth are provided and the three output conductors. The three sets of teeth would be equally spaced inthe same fashion as the two phase embodiment shown.

It will be seen that the device of Figure 7 provides two phases 9.0" out of phase with one another. Two phases out of phase can be obtained by providing a second output conductor in the device of Figure 1. The second conductor would be wound around the teeth 12 through 17. 'l hewindings would be in opposite directions to one another. r

It will also be In Figures 8 and 9 several rather interesting embodiments of the invention are depicted. The fixed teeth and the inner section of the ferromagnetic core are modified. The outer portion 11 remains. A motor 59 is provided. The motor drives a shaft 60. A central or inner portion of the ferromagnetic core 61 is carried by the shaft. First and second ferromagnetic teeth 62, 63 are connected to the inner portion 61 of the core. The inner portion of the core 61 and the teeth 62, 63 rotate together with the shaft 60. Output windings 64, 65 are provided and connected to slip rings 66, 67.

Input windings. 68, 69, 70 are provided to excite the ferromagnetic core. The input windings shown in Figures 8 and 9 are three phase two pole corresponding to the input windings of Figure 1.

It willbe seen that with this device an almost infinite number of output frequencies can be obtained. If the shaft 60 rotates at the same speed as the rotating field it will be seen that no output will be induced. If the shaft 60 rotates in the same direction but more slowly than the rotating field then the output frequency will be reduced, as compared with the input frequency. If the shaft 60 rotates in the opposite direction to the rotating field then the output frequency will be materially increased as compared with the input frequency.

Thus by in effect replacing the inner core portion and teeth of Figure 1 with a rotating inner core portion 61 and rotating teeth 62, 63 we obtain a device wherein the scope of frequency change is practically infinite. If the motor speed is zero the output frequency would be the same as the input frequency. The output frequency may also be greater or less than the input frequency depending on the speed of the motor. The number of input poles, magnetic paths, and output phases have the same arrangement as described in the stationary frequency multiplier of Figures 1 through 7.

In addition to the foregoing advantages the device of Figures 8 and 9 also provide a variable frequency without complex, expensive, and sensitive equipment. Further, the output is not restricted to an odd multiple of the base frequency. With a synchronous driving motor the output frequency isa direct multiple of the input frequency and therefore frequency regulation is as good as the input frequency. The unit is relatively simple, small, and rugged as compared with prior'known variable frequency changing devices. Thedevice is capable of providing low frequencies which is extremely difficult with prior known methods. It also can be constructed to convert rather large amounts of power for a given size unit and at a high efliciency of conversion.

In Figures IO'through 12 a modification of this invention is shown which is a frequency changer capable of changing alternating current to a lower, a higher, or a varied waved shaped alternating current or to direct current. In this latter sense it is actually a rectifier which changes alternating current to direct current in relatively large blocks of power. It does not have the limitations of prior known rectifiers.

This modification of Figures 10 through 12 has further advantages. It is rugged, simple and inexpensive. It is essentially a stationary device in a sense that the rotating parts are very small, An AC. motor driving'aDC. generator to rectify comparable electrical forces would require two rather massive rotating bodies as compared with the one'small rotating body of this invention. Further, the disclosed device has a much higher efliciency of conversion than prior known mechanisms for changing A.C. current to D.C. It can be constructed for practically any size load. In other words, one of these frequency changers can be used in place of a rectifier or a large motor generator. Thus in an application where direct current loads are sometimes low and sometimes high un der prior known techniques .both a rectifier and a motor generator or a stack of rectifier'smight be provided. With this device the rectifier and motor generator or the stack 'of rectifiers are replaced by one unit which is simpler than either of the prior known devices.

Further, the device of Figures 10 through 12 can be used in combination with the device of Figures 8 and 9. The device of Figures 10 through 12 can be used to tap off a sufficient amount of current to provide a direct current drive for the motor 59 of Figures 9 and 10. A very accurate "control of the speed of the motor 59 can be obtained in this way which provides adjustment of the current in D.C. motor windings.

The device of Figures 10 through 12 is a modification of the device of Figure 1. The central or inner core 10' has a bore 74 with a smaller concentric bore formed in it. A shaft 72 is carried in the bores and driven by a synchronous motor 73. The shaft 72, the inner ferromagnetic core-10' and the outer core 11 are coaxial.

The motor 73 is in reality simply a permanent magnet. It is caused to rotate by the rotating magnetic field induced in the surrounding ferromagnetic elements. The motor or magnet 73 is disposedwithin the bore or recess 74.

Slip rings 75, 76 are carried by the shaft 72. The output conductor 30 is connected to brushes 77, 78 which in turn are in electrical contact with the slip rings 75, 76 respectively. First and second split output slip rings are also provided. The first split slip ring is comprised of the slip ring sections 80, 81 and the second split slip ring is comprised of the slip ring sections 82, 83. The slip ring sections 80, 83 are connected together by conductor 84. They in turn are connected to the slip ring by the conductor 85. The slip ring sections 81, 82 are connected together by conductor 86. These slip ring sec tions 81, 82 are in turn connected to the slip ring 76 by conductor 87.

As the north magnetic pole of the rotating field rotates clockwise from the position indicated by A in Figure 10 to the position indicated by the letter B, three positive half cycles are developed. As the north magnetic pole rotates from the position 13 to the position A three negative half cycles are developed. Through the use of the split slip rings, the polarities of the output connections are reversed each l of revolution of the shaft 72 and therefore the half waves are always positive. The output of this device is shown in Figure 13 where it will be seen that a full wave rectified D.C. current is obtained.

It will 'be recognized that any reliable method of reversing the polarity ofthe output windings at the proper time may be used. It is also possible to obtain the same effect by controlling the polarity of the input windings. For example, this could be accomplished by two polyphase power'sources out of phase. These sources may be produced by a phase shifter. With such a construction the north magnetic pole rotates from A to B and then returns to A. In such a construction the first polyphase system would create a rotating field clockwise from'A to B and then be disconnected. The second phase would then create a comparable field from A to B and be disconnected and so on.

A more nearly constant D.C. voltage may be obtained by providing agreater number of magnetic paths. As an example, a frequency changer for direct current could be made with windingscorresponding to those disclosed in Figure 7, but with 18 teeth and 3 output conductors. This would provide 18 magnetic paths with 3 output phases. The output phase relationship of such a device is shown in Figure =14. If connections are made in accordance with the teaching of Figures 10 through 12 and the 3 output phases are connected in series, a resultant D.C.volta-ge such as that shown in Figure 14 is obtained. The output of this device may be filtered to reduce the rippleby any of the well 'knownprior techniques. It will be noted that generally speaking the greater the number of magnetic paths the more nearly a pure D.C. voltage is developed.

From the foregoing d escription it will be seen that the designer has a great deal of latitude in selecting the output from one of these devices. The designer has control over frequency, phase relationship, voltage, and polarity. It will be seen that many modifications of the basic design may be selected to achieve any desired result. As an example,;Figure shows a voltage which approaches .a square wave. The frequency changer is constructed so that one pair of output coils develops section D, another section E, and another section F. The polarity then reverses and sections G, H, and J are developed. Thus, a square wave is established.-

Yet another example is shown at Figure 16. There, a saw toothed wave is obtained by arranging two sets of coils in such a manner that the two voltages developed provide the resultant wave shown. One set of coils is arranged to always give a positive voltage and the second set to always anegative voltage. These voltages are separated in time phase by 90 to provide the saw toothed voltage as a resultant. Thus a designer may construct a device to change alternating current to any desired electrical output.

There has thus been described an improved electric wave changer in which nonsaturated magnetic paths are provided to form a device which is quiet, eflicient, cool,

and simple as compared to prior frequency multipliers.

Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

1 claim:

1.A frequency changer comprising, first and second ferromagnetic portion elements, said portions being a central ferromagnetic portion and a surrounding outer ferromagnetic portion, a plurality offerromagnetic teeth elements extending radially between said portions, said teeth being connected to atleast one of said portions, input windings disposed about said first portion to induce ar'otating magnetic field, said input windings being of an appropriate size to induce a non-saturated magnetic field throughout said elements, and an output conductor, said conductor being wound in the field of magnetic infiuence of each of said teeth to provide connected output windings, the output windings on alternate teeth .being in opposite directions. i

2. A static alternating current wave changer having a ferromagnetic core consisting of two body portions connected by a path defining portion, said path defining portion containing spaced ferromagnetic teeth extending transversely thereof, exciting windings for generating a rotating magnetic field within said core with the magnetic flux extending longitudinally of the teeth and. the field as a whole and moving transversely to the teeth and lengthwise of the path defining portion, a path unit containing the following for one output phase; a series of ferromagnetic teeth elements forming an odd number of pole pairs, the exciting field creating apair of conjugate unsaturated zones, the teeth in one of the unsaturated zones having a magnetic polarity opposite to that of the teeth in the other of the saturated zones, unsaturated zones terminating at two magnetic. neutral regions with the two neutral regions spaced 180 electromagnetic de-' grees apart, each of said unsaturated zones-including substantially no more. than one entire pole at one time, winding elements linking all teeth, each winding element having a cooperating tooth .and become active for generating action when its cooperating tooth is swept by one of said second pair of unsaturated zones, and means for connecting the winding elements corresponding to one pole pair in series with the winding elements corresponding to the other pole pairsto. output terminals to provide a potential at a multiplied frequency whose stepped up ratio is equal to the odd number by said teeth.

- 3. A static alternating current wave changer having an inner ferromagnetic core, an outer annular ferromagnetic core surrounding said inner core, a set of ferromagnetic teeth symmetrically disposed and extending between said inner core and said outer core, the teeth forming an odd number of pole pairs, a coil for each tooth functioning as an output winding element, means for connecting said coils to output terminals with coils for adjacent poles reversed in polarity, and polyphased windings disposed on said inner core, said polyphased windings providing an exciting field whose flux at any instant extends from theinner core along some teeth to the outer core, along said outer core and alongother teeth back to the inner core, said ferromagnetic cores and teeth being unsatu rated, said exciting windings creating a rotating exciting field in said cores so that potential is induced in the coil to provide an alternating potential at a frequency stepup corresponding to said odd number of pairs of teeth. 4. A frequency changer comprising, first and second ferromagnetic portion elements, said portions being a central ferromagnetic portion and a surrounding outer ferromagnetic portion, a plurality of ferromagnetic teeth elements extending radially between said portions, said teeth being connected to at least one of said portions, input windings disposed about said first portion to induce a rotating magnetic field, said input windings being of an appropriate size to induce a magnetic field throughout said elements, an'output conductor, said conductor being wound in the field of magnetic influence of each of said teeth to provide connected output windings, said inner portion having an axial bore formed therein, a shaft carried in the bore, a plurality of slip rings carried by the shaft, said output conductor being connected to said slip rings, and means to cause relative rotation of said shaft and slip rings and said outer portion. 5. A frequency changer comprising,,-first and second ferromagnetic portion elements, said portions being a central ferromagnetic portion and a surrounding outer ferromagnetic portion, a plurality of ferromagnetic teeth elements extending radially between said portions, said teeth being connected to at least one of said portions, in.- put windings disposed about said .first portion to induce a revolving magnetic field, said input windings being'of an appropriatevsize to induce a magnetic field throughout said elements, an output conductor; said conductor being wound in the fieldof magnetic influence of each of said teeth to provide connected output windings, said inner portion having an axial bore formed therein, a shaft carried in the bore, a plurality of slip rings carried by the shaft, said output conductor being connectedto said slip rings, means to cause relative rotation of said shaft and sliprings and said outer portion, and means to reverse the polarity of the output slip rings upon each degrees travel of the rotating magnetic field. V

6. A frequency changer comprising, first and second ferromagnetic portion elements, said portions being a central ferromagnetic portion and a surrounding outer ferromagnetic portion, a plurality of ferromagnetic teeth elements extending radially between said portions, said teeth being connected to at least one of said portions, input windings disposed about said first portionto induce a revolving magnetic field, said input windings being of an appropriate size to induce a magnetic field throughout said elements, an output conductor, said conductor being wound in the field of magnetic influence of each of said teeth to provideconnected output windings, said inner portion having an axial bore formed therein, a shaft carried in the bore, a plurality of slip rings carried by the shaft, said output conductor being connected to said-slip rings, means to cause relative rotation of said shaft and slip rings and said outer portion, first andsecond slip ring Sections forming a first split slip ring, and third and fourth ofrpole pairs formed.

slip ring sections forming a second split slip ring, the split slip rings being carried by said shaft, the first and third slip ring sections being electrically connected together and to one of said slip rings, the second and fourth slip ring sections being connected together and to another of said slip rings, whereby to provide a device in which the polarity of the induced output is reversed for each 180 degrees rotation of the revolving magnetic field.

7. A frequency changer comprising, first and second ferromagnetic portion elements, said portions being a central ferromagnetic portion and a surrounding outer ferromagnetic portion, a plurality of ferromagnetic teeth elements extending radially between said portions, said teeth being connected to at least one of said port-ions, input windings disposed about said first portion to induce a revolving magnetic field, said input windings being of an appropriate size to induce a magnetic field throughout said elements, an output conductor, said conductor being wound in the field of magnetic influence of each of said teeth to provide connected output windings, said inner portion having an axial bore formed therein, a shaft carried in the bore, a plurality of slip rings carried by the shaft, said output conductor being connected to said slip rings, a permanent magnet carried by said shaft, said magnet forming a synchronous motor caused to rotate by said revolving field, and means to reverse the polarity of the output slip rings upon each 180 degrees travel of the rotating magnetic field.

throughout said elements, said teeth being arranged in groups, there being one group for each output phase, and an output conductor for each output phase, each output conductor being associated with one group of teeth, each such output conductor being wound in the field of influence of each tooth of the associated group to provide connected output windings.

References Cited in the file of this patent UNITED STATES PATENTS 1,955,133 Kovacs Apr. 17, 1934 2,470,975 Bussey May 24, 1949 2,611,119 McCreary Sept. 16, 1952 2,790,131 Nyyssonen Apr. 23, 1957 FOREIGN PATENTS 565,266 Great Britain Nov. 2, 1944 

